Figure 6.

Schematic model of the outer gating process. The two TMDs are depicted as half‐cylinders. The NBDs are shown as shorter half‐cylinders, which hold two molecules of ATP (grey‐shaded ovals) bound at their interface. The domain swap of TMDs and NBDs is indicated by a rotation of the TMDs relative to the NBDs. The membrane is symbolized by a wire frame box. The two outer gates are shown as colored disks. A functional outer gate is indicated in green, while a dysfunctional gate is shown in yellow. The two symmetry‐related rh123 binding modes are symbolized by dotted ovals (green lettering in normal or mirror writing). Arg + refers to binding‐mode selector residues introduced in symmetry‐related positions 132 (right) or 773 (left). An X indicates that the respective binding mode was deselected by introduction of the respective arginine residue and that the corresponding gate is not used (no arrow). (A) Wild‐type P‐gp. Depending on its binding mode, rh123 may exit the binding pocket via outer gate 1 (left) or 2 (right). (B) Mutant Y310A: Gate 1 is dysfunctional, a facilitative (defective gate 1) and active transport component (functional gate 2) are present simultaneously. (C) Mutant Y953A: Gate 2 is dysfunctional. Again a facilitative (defective gate 2) and active transport (functional gate 1) component superimpose. (D) Mutant Q132R: rh123 binds in mode 1 only and exits via gate 1. (E) Mutant Q773R: rh123 binds in mode 2 only and exits via gate 2. Both mutants show active transport characteristics.8 (F) Mutant Y310A/Q725L/Q132R: rh123 binds in mode 1, outer gate 1 is dysfunctional. The mutant shows facilitative behavior, as indicated by a bidirectional arrow. (G) Mutant Y953A/Q132R: rh123 binds in mode 1 and exits via a functional gate 1. Active transport characteristics are retained, as the dysfunctional gate 2 is not used. (H) Mutant Y310A/Q725L/Q773R: rh123 binds in mode 2. Active transport is observed, as dysfunctional gate 1 is not used. (I) Mutant Y953A/Q773R: P‐gp adopts facilitative behavior, as rh123 binds in mode 2 and gate 2 is dysfunctional. (J) the double Arg + mutant Q132R/Q773R does not show rh123 transport8